Transponders of many shapes, styles, form-factors, and costs are well known in the art. Many of the earliest transponders were primarily deployed for military tracking applications, such as IFF (interrogative friend or foe) applications other target-monitoring applications. Commercial transponders (often used for security-oriented applications and/or for presence-monitoring applications) are also well known in the art.
Many of the earliest commercial security-oriented and/or presence-monitoring applications—i.e., “presence detection application systems”—use a frequency divider as an electronic tag in typical “anti-shoplifting” commercial transponder applications. Such systems typically include (1) a transmitter for transmitting a scanning signal at a first frequency in a surveillance zone; (2) an electronic tag including an active frequency divider for detecting electromagnetic radiation at the first frequency and for transmitting a presence signal in response thereto at a second frequency that is a sub-multiple of the first frequency; and (3) a receiver for detecting electromagnetic radiation at the second frequency to thereby detect the presence of the electronic tag in a “surveillance zone”. In such applications, the electronic tags are attached to articles of which detection is desired, to enable surveillance/detection of the presence of such articles, usually at fixed “surveillance zones”. Such presence detection systems are proven highly useful for controlling flows of tagged items past “choke points”, e.g., for detecting shoplifting at ingress/egress points at department stores, high-security areas, etc.
A few examples of other related applications include: (1) detecting the presence of a person or vehicle carrying an electronic tag in a surveillance zone (e.g., automatic payment systems for toll booths); (2) detecting the presence of articles bearing electronic tags within a surveillance zone along an assembly line (inventory monitoring, management, and control); (3) detecting the presence of keys attached to electronic tags in a surveillance zone at the exit of an area from which such keys are not to be removed (high security checkpoints), etc. For any of these implementations, the electronic tags can be encased in small card-shaped, snap-shaped, “vice-grip”-shaped, or other form factor electronic packages (such as “removably attachable” devices) that can be attached to a target article in such a manner that the tag cannot be removed from the article without a special tool.
When used in a shoplifting detection system application, e.g., a sales clerk uses such a special tool to remove the electronic tag from merchandise that is paid for. The surveillance zone is located near the doorway, to enable detection of articles from which the electronic tags have not been removed. Some such applications include complex frequency dividers that must be powered by expensive long-life miniature batteries. Other prior art frequency dividers also use either a battery or an external power supply. Most such presence detection systems are neither small, nor portable, nor low in cost.